PUVA (psoralens plus ultraviolet A) is an effective therapy for psoriasis but often has systemic side-effects. We have been testing whether bath water delivery of psoralens in conjunction with UVA is a safe and effective alternate route of therapy for psoriasis Systemic absorption of psoralen from bath water appears to be minimal, virtually eliminating potential systemic side effects. Bath water psoralens also produce high skin levels of this photosensitizer and the need for less UVA, thus possibly decreasing the risk of skin carcinogenicity. We are now (1) studying the efficacy of Trioxsalen bath water PUVA therapy in the treatment of psoriasis and (2) investigating the mechanism of action of Trioxsalen bath water PUVA therapy (3) comparing it to our studies with bath water PUVA using Methoxsalen, where we treated 55 patients and the efficacy was good to excellent in approximately 80% of patients treated.